This invention relates to a sucker rod coupling pin prepared from fiber-reinforced plastics material.
Until now, any device used in the drilling of an oil well has been fabricated exclusively from steel. This practice is accompanied with the drawback that steel is a heavy material and is, moreover, very susceptible to corrosion. In recent times, the quality of fiber-reinforced plastics material has greatly improved. This material is lightweight and is highly corrosion-proof, and thus has come to be more widely used for manufacturing parts of oil well-drilling equipment.
When a deep oil well is drilled, the parts are used in the form of a number of connected units. A coupling type sucker rod is usually fabricated by fitting a metal pin to the rod end. The pins are joined together by a threaded coupling.
However, the threaded coupling has, in the past, presented difficulties with regard to mechanical strength when used to effect the connection of metal pins to the sucker rod. Consequently, various devices have been proposed to resolve these difficulties. One of the proposed processes, which consists of threading the outer wall of the rod itself, tends to cut up reinforcing fiber, resulting in an unacceptable decline in the mechanical strength of the coupling. At present, therefore, a friction type coupling dispensing with threading is being developed.
In this connection, metal coupling pin 1 will now be described with reference to FIG. 4. This coupling pin 1 comprises a coupling member 2 which is provided with elongated hole 4 into which sucker rod 10 is inserted. Threaded section 3 is formed on the peripheral portion of coupling member 2 at the closed end of coupling member 2 to be engaged with external coupling means (not shown). Until now, a metal coupling member 2 has been employed. Three annular grooves are so formed as to surround the inner wall of coupling member 2. As viewed crosswise, the three annular grooves each have two tapered surface portions. Tapered surface portions 7, lying nearer to the closed end of hole 4, have a sharply rising plane, toward the open end of hold 4. Tapered surface portions 6, positioned remote from the closed end of hole 4, have a plane inclining gradually downward toward the open end of hole 4. Upward and downward inclining surfaces 7 and 6 are alternately arranged three times. Annular surface portion 5, surrounding the inner periphery of the open end of hole 4, has a long flat plane and is spaced from the outer surface of rod 10.
Resin 30 is placed in elongated hole 4, and rod 10 is forced thereinto. The resin is then allowed to harden, tightly gripping rod 10. A rod-pulling force is transmitted by means of the above-mentioned tapered depressions defined by surfaces 6, 7, with the aid of friction between the resin and the surfaces 6, 7.
The outer diameter of coupling member 2 progressively decreases toward the open end of elongated hole 4, thereby preventing an excessive force from being applied to sucker rod 10 if it happens to become warped. However, the proposed coupling pin 1 of FIG. 4 has the drawbacks in that since the length l of the tapered surfaces 6 and taper angle .alpha. thereof have the same value, contact pressure between coupling pin 1 and rod 10 varies more noticeably in the axial direction. When, therefore, sucker rod 10 is drawn out of elongated hole 4, a strong frictional force is generated at the second taper section as counted back from the open end of elongated hole 4, frequently causing sucker rod 10 to break at said second taper section.
The present invention has been developed in view of the above-mentioned circumstances, and is intended to provide a sucker rod coupling pin the design of which resolves the aforementioned difficulties.